The production of chemical pulp for paper production chiefly uses wood as a raw material. However, about 10% of the chemical pulp produced the world over is produced from other plants and plant residues such as grass, reed, straw and bagasse. The characteristic of these raw materials is that they contain, among other substances, considerably more silicon than wood does.
Chemical paper pulp is produced by cooking the raw material with chemicals, non-fibrous material being then chemically dissolved. The cellulose fibres constituting the paper pulp will remain. A large part of the dissolved material consists of lignin, but also of other organic and inorganic substances. The production of chemical pulp from wood mainly implements the sulphate process, in which sodium hydroxide and sodium sulphide are active components of the cooking liquor. For the cooking of non-wood raw material sodium hydroxide alone is required as an active chemical, and for this reason the major part of chemical pulp production from these raw materials is performed with the process called the soda process.
In the soda process the raw material is heated together with an alkaline cooking liquor containing sodium hydroxide to a temperature in the range from 140 to 170.degree. C. under pressure. The cooking liquor should have a high alkaline concentration. Under these conditions, the main portion of lignin will be dissolved from the raw material, however, also the main portion of the silicon in the raw material will react with the sodium hydroxide, forming water-soluble sodium silicate. Thus the black liquor produced in cooking will contain silicate ions in addition to lignin and other organic compounds.
On principle, sodium hydroxide can be recovered from the black liquor and the organic substance in the black liquor can be utilised as a fuel for energy generation. Contrary to the sulphate process, in which the cooking liquor contains sulphide ions and the combustion of black liquor requires a chemically reducing environment in the bottom section of the combustion equipment, the black liquor obtained by using the soda process can be burnt even under highly oxidising conditions. Chemical recovery is performed by evaporating the black liquor to a suitable dry matter content and by burning the evaporated liquor by means of excess oxygen. The inorganic combustion residue, consisting mainly of sodium carbonate, is dissolved in water and causticised with burnt lime to form sodium hydroxide, which in turn is used in pulp cooking. This is the practice in the case of black liquors poor in silicate. However, the presence of silicate in the black liquor entails serious problems regarding recovery. The main problems consist in scaling of the heat exchange surfaces of the evaporator in which the black liquor is concentrated, high viscosity of the concentrated black liquor, which impedes liquor combustion, and forming of calcium silicate in the lime used for converting the sodium carbonate to sodium hydroxide. The calcium silicate makes recycling of the lime difficult or quite impossible. Due to the problems caused by silicate, pulp mills using e.g. straw as a raw material are unable to reclaim the black liquor and to recover sodium hydroxide from this, but instead discharge the black liquor. This is evidently a heavy financial burden for the mill and very detrimental to the environment around the mill.
There have been attempts to solve the problems caused by silicate in the black liquor over the past 20 years. It is a well-known chemical fact that, when the acidity, i.e. the pH of the solution is reduced, the silicate ions will form a partly gelly and partly crystalline silica (cf. e.g. Chemical Abstracts, Vol. 89, No 61292, 1978). Methods for removing silicate from black liquor have been developed on the basis of this crystallisation. A process following the procedure described here was installed at the RAKTA company's mill in Egypt. The pH reduction is performed by introducing flue gas containing carbon dioxide into the black liquor. In accordance with the described procedure, the substance containing silicon is separated from the black liquor by sedimentation and/or centrifugation, after which the silicon is precipitated as a mixture of silica and calcium silicate by adding burnt or slaked lime. The process has been patented by J. Mulder and P. Gutmann (Germany, 3003090 8/1981).
All the same, the RAKTA installation never worked satisfactorily in the practice. The reason for this is that, in order to convert a sufficient portion of the silicate into a crystalline and hence separable form, the pH must be so much decreased that also a portion of the lignin in the black liquor will be converted into a gelatinous phase and that makes the separation of the silica and the gelatinous lignin from the liquor very difficult. Removal of lignin from the liquor will decrease its calorific value, so it will not be high enough to allow it to be burnt in steam boilers developed for liquor combustion. Another attempt to enhance the techniques developed in the late 70's is being made at a mill in Tamil Nadu, India. This process still follows the same principle as the RAKTA process. The operation aims at recycling crystal nuclei of silica at a pH high enough to selectively precipitate silica without coprecipitation of lignin.
Another means of removing silicon from the chemical recovery cycle at a mill which produces paper pulp from straw following the soda process is described in the Indian patent 17 1289 from 1989. In this process, the silicate-containing black liquor is evaporated to an approx. 40% dry matter content, after which it is mixed with dry organic material, e.g. rice husks, the ensuing dry matter content being approx. 60 per cent. The mixture is burnt on a chain grate in the furnace of a steam boiler. The inorganic combustion residue, containing mainly sodium carbonate and sodium silicate, is dissolved in water, and flue gases containing carbon dioxide are introduced in the aqueous solution. The carbon dioxide reduces the pH, and then the silicate forms poorly soluble silica, which is filtrated out of the solution. In this manner, the silicon can be removed without the lignin interfering with the separation of the precipitate or it losing its energy content. The process of the patent involves the problem of requiring the combustion temperature to be maintained within the range from 750 to 820.degree. C. to prevent the ashes from melting and to simultaneously completely burn the organic substance completely. These aims are considered to be very difficult to achieve at a full-scale plant, and the process has not been industrially implemented so far.
U.S. Pat. No. 4,504,356 also discloses a process for removing silicate from black liquor, in which the black liquor is first evaporated and is subsequently treated with flue gases, which are supplied in an amount of 30 to 40 m.sup.3 per m.sup.3 of black liquor. The precipitated silica is separated, diluted and washed with water. However, the evaporation of the silicate-containing black liquor causes problems due to scaling etc., and the problems with co-precipitation of lignin are not overcome.
A further inconvenience of these known processes is that flue gases are used as the carbon dioxide source. Flue gases are obtained in the combustion of an organic substance by means of air and they contain 10 to 20% by volume of carbon dioxide, whereas the major portion of the gas consists of the nitrogen in the combustion air. This inert portion of the flue gas will not be absorbed by the alkaline solution containing silicate, but will instead pass through this and cause foaming.
The purpose of the present invention is to eliminate the drawbacks of these known processes and to provide a process for economical and trouble-free recovery of alkali and heat from black liquor containing silicate in an environment-friendly manner.
The main characteristics of the present invention are set forth in the accompanying claims.